1. Field of the Invention
The present invention relates to a position indicator, such as a stylus pen, capable of detecting operational information represented by a continuous variable and transmitting the detected operational information to a tablet.
2. Description of the Related Art
Examples of known position indicators are disclosed in Japanese Unexamined Patent Application Publication No. H07-175572, corresponding to U.S. Pat. No. 5,679,930, incorporated by reference herein. According to Japanese Unexamined Patent Application Publication No. H07-175572, operational information represented by a continuous variable, such as pen pressure, drawing line width, color/darkness, etc., is converted into a time period by a time-constant circuit. During the converted time period, the number of AC signal waves transmitted from a tablet is counted and encoded into a binary code, which is indicative of the detected continuous variable, to be sent back to the tablet. With such position indicators, the distance and positional relationship between the tablet and the position indicator do not affect detection of the continuous variable, and the continuous variable can be accurately detected with relatively low power consumption.
However, to further increase the accuracy and resolution of detection of a continuous variable, either the frequency of the signal to be transmitted from the tablet has to be increased, or the continuous variable detection time has to be lengthened. If the frequency is increased, the pen-tablet device as a whole will become expensive, and also the power consumption will be increased. If the continuous variable detection time is lengthened, detection number per unit time (sampling speed) will decrease.
To address the aforesaid challenges, according to Japanese Unexamined Patent Application Publication No. 2005-165768, corresponding to U.S. Patent Application Publication No. 2005/0127893, incorporated by reference herein, a continuous variable can be detected in a short period of time in high resolution, by measuring a time constant that can be switched between two types (or ranges) in a time-constant circuit including an element whose characteristic changes in response to the operational information (e.g., pen pressure).
Specifically, according to Japanese Unexamined Patent Application Publication No. 2005-165768, in order to predetermine the timing for switching between the two types of time constant, a rough measurement based on a smaller-value time constant is first performed before an actual measurement based on the two types of time constant is performed. The step of switching the time constant from a smaller value to a larger value, to thereby perform a high-resolution measurement, is carried out at a point when a time period slightly shorter than the predetermined period has elapsed. With such arrangement, a continuous variable can be detected in a short period of time and in high resolution.
However, according to Japanese Unexamined Patent Application Publication No. 2005-165768, since a measurement involving charging or discharging is performed twice (during the switching-timing predetermination measurement and an actual measurement), longer time is required, and further, the sampling speed will decrease if detection is performed in high resolution.